The present invention relates to an image forming apparatus and a production method thereof, which are applied to an electrophotographic copier, a laser beam printer, or similar equipment in which an image is written by a laser beam.
Recently, an electrophotographic copier, printer, or similar equipment in which an image is written by a laser beam, is much in use, and an image forming apparatus, which can conduct higher speed copying, is required for the market. However, when the speed of the image forming apparatus is made higher, it is difficult to achieve a high speed because an image is written on each line by laser beam scanning. To achieve a higher speed, although it is required to stably rotate a polygonal mirror, which controls the optical scanning speed at a higher rate, it is difficult to achieve the higher speed with the present mechanism of the image forming apparatus.
For this purpose, the following has been proposed: a plurality of laser beams are simultaneously used for scanning to increase the writing speed more than two times.
However, when a plurality of laser beams are used for scanning, a difficult problem occurs in the positioning of a primary scanning direction and a subsidiary scanning direction with respect to each other. Specifically, due to some deviation of one laser beam to the other on an image, irregularity occurs on the peripheral portion of the image, and reproducibility of fine lines, image density, gradation reproducibility, and a degree of resolution become poor, so that a high quality image can not be provided. Although the wavelength of a semiconductor laser beam is monistically determined by semiconductor material in principle, a slight difference of the wavelength occurs due to fluctuations of its composition or its crystal system. For this reason, when an image is written by a plurality of laser beams, this slight fluctuation of the wavelength largely adversely affects the image writing.
That is, when a plurality of laser beams pass through the same optical system for scanning, the beams are required to pass through various types of lenses for appropriate exposure scanning. However, when the wavelength is different from each other, the beam's speed passing through the lenses is different from each other. In this case, when the plurality of laser beams are used for exposure scanning, the arrival time is different from each other. Accordingly, the length of the exposure scanning in the primary scanning direction is different, and the writing position in the direction of the primary scanning is different, resulting in image deviation. Further, also due to the difference of the refractive index of each laser beam due to aberration of an f.theta. lens, the phenomena of deviation in the primary scanning direction is amplified. This deviation of the image results in a deviation of 4 .mu.m on a image per 1 nm of the difference of the wavelength. Accordingly, when a plurality of laser beams having fluctuations in the wavelength are arbitrarily combined with each other, the difference of the wavelength of laser beams is 30 nm at the maximum, and the deviation is 120 .mu.m on the image. Thereby, irregularity occurs on the peripheral portion of the image, and reproducibility of fine lines, image density, gradation reproducibility, and resolution become poor, so that a high quality image can not be provided.
Accordingly, it is preferable to manufacture a laser with small fluctuations in the production of the laser itself. Because the wavelength of oscillation of the semiconductor laser is mainly determined by the width of forbidden band of active material included in a mixed crystal, it may be possible that crystal is formed by strictly controlling the component, in principle. However, it is almost impossible to perform the above-described production in practice.